The present invention relates to liquid injection molding machines and more, particularly the present invention relates to a plunger type of liquid molding machines useful for injecting or utilizing liquid silicone composition.
Injection molding machines provide a unique advantage over other types of molding machines in preparing molded or encapsulated parts. Accordingly, molding is much simpler than with other type of systems. For instance, for molding of elastomers normally require several distinct operations, such as milling, sheeting or extruding, compression, transfer or injection molding followed by deflashing of the finished product. Traditionally there has been two types of injection molding machine. One is the recipricating screw injection molding machine as disclosed on improvement thereover which is disclosed in the patent docket of Aldo Laghi Docket 60SI-308 filed June 13, 1980 entitled Apparatus For Liquid Injection Molded Parts. Prior to the modification of such machine pellets were inserted into a barrel where a screw picked up the pellets by force causing them to liquidify and force them in front of a ram in a barrel means. The ram would push at the appropriate time the liquified plastic out of a nozzle into a mold in front of the nozzle and the screw would retract from the mold leaving the molded part to be removed from the mold by known techniques. The improvement of A. Laghi in the copending docket was to include sealing means at the end of the screw means so that the liquified liquid molding composition would not leak out of the rear of the barrel means of the machine. Another modification was the utilization of a value to shut off the feed means into the barrel means of the liquid composition prior to the forcing of the liquid compositions into the mold by the screw means. The plastic pellets were feed into the barrel of the molding machine through a hopper and they were liquified by the forcing action of the screw forcing the material forward of the front end of the screw in the barrel means. Accordingly, when the screw moved forward it was just the pressure of the screw applied on the liquified organic cmposition to force it into the mold. However, at the same time the pressure that was utilized in the feed means to force the liquified composition into the barrel means was shut off and did not affect the liquid composition in the barrel means. Accordingly, the logic of the control valve of the copending Laghi docket was utilized to control the amount of liquified composition that would be introduced into the barrel means in front of the the ram of the screw in the barrel means and the valve would close and the only pressure affected upon the liquified composition barrel means would the pressure of the screw moving forward.
These modifications and improvement to the traditional reciprocating screw injection molding machines was made in the copending docket to allow the paresent machines to utilize liquid compositions and more specifically silicone liquid composition. It should be noted that the foregoing modifications of that patent application as well as the present application can be utilized with any liquid injection molding composition including a liquid silicone injection molding composition. The silicones have many advantages for injection molding compared to most organic elastomers. They cure more rapidly, they are easily pumpable, immiscible, provide easy release from the molded surface and in most cases do not required deflashing of the molding parts. Accordingly, the actual injection molding can be performed in cycle times which are much shorter than with other elastomers and the additional steps required for the molding of the traditional elastomers is substantially reduced. The milling of sheet or extruding operations are not required because of the liquid nature of the silicones which also permits accurate measurements and delivery to conventional pumping system. The deflasing operation is also eliminating due to the low cavity pressure and the cure characteristics of the material. As a result, considerable savings of labor overhead and material may be obtainable by switching from conventional molding processes to liquid injection molding. In fact, in some case when production volume is large and the size of the molding parts are small, liquid silicones can be more than cost competitive with even the cheapest organics. This is not surprising considering the cost of some of the steps of conventional molding processes such as molding and deflashing which depends most on the number of parts that are molded and not at the amount of material used. Liquid injection molding is characterized by a small capital investment because a small injection press can produce large number of parts per year in the low cavity pressure in fast cycle times. It should be noted that the properties of silicones add to the quality of the molded part which properties of silicones are in the part the superior or elastomeric properties of silicones at high and low temperatures and that they resist aging better than most other elastomers and plastics and they offer superior dielectric characteristics.
Various silicone compositions have been developed for the use of liquid injection molding machine. Basically such compositions comprise a vinyl containing polysiloxane, a hydride polysiloxane as opposed to a cross-linking agent and a platinum catalyst. Such compositions are traditionally referred to in silicone chemistry as SiH olefin platinum catalyzed compositions or addition cure composition or addition cure RTV compositions. Such compositions may contain various other ingredients such as for instance fillers, pigments and so forth. For example, fillers are fumed silica, precipitated silica, treated or untreated, and also extending fillers such as lithopone, zinc oxide, glass fibers, alpha quartz, and magnesium oxide and other type of fillers. Such compositions are sold in two forms for liquid injection molding compositions. In the one component form such compositions comprise the above ingredients with other additives as stated and in addition an inhibitor, where the inhibitor is preferably a hydroperoxy compound as disclosed in Bobear U.S. Pat. No. 4,061,609. These hydroperoxy inhibitors are very effective inhibitor which allows the composition to be mixed into a single component without curing for periods of time from six months to two years which at elevated temperatures cures rapidly in the matter of seconds. By elevated temperatures it is meant above 100.degree. C. However such compositions are also made in two components. In such two component compositions the hydride polysiloxane is kept separate from the vinyl polysiloxane and the platinum catalyst is packaged in one or the other components. The materials is packaged in two components or two packages such that there is not a component or package in which there appears all three ingredients, that is the vinyl siloxane, the hydride cross-linking agent and the platinum catalyst. Such compositions which are into two components or two packages when the two packages are mixed will cure slowly at room temperature to a silicone elastomer and will cure rapidly at elevated temperatures to also a silicone elastomer. The shelf life of the composition when the two components are mixtued together can be extended, that is preventing the composition from curing when the two packages are mixed at room temperature can be extended for periods of days and even weeks by utilizing various types of inhibitors. Accordingly, there can be a long pot life or mixing time of the components and yet when the components are inserted to the molding and heated at elevated tempertures about 100.degree. C. the composition will cure to a silicone elastomer in a matter of seconds. An example of two component compositions or inhibitors for two component compositions is, for instance, that disclosed in the patent application of Eckberg, Ser. No. 40,015. Another type of inhibitor is a low molecular weight silanol containing polysiloxane having from about 5 percent by weight of silanol. On an alkyl isocyanurate as disclosed in the patent application of Eckberg, Ser. No. 81,966. Depending on which inhibitor is used either a one component or two component composition silicone composition will result which can be utilized in a liquid injection molding machine of the types disclosed in the foregoing Laghi copending application and in the instant patent application. With respect to whether one component or two component silicone compositions are utilized, either one may be utilized with the advantages as pointed above that liquid composition have for an injection molding machine. Accordingly, it becomes desirable to utilize such compositions with a plunger type or ram type of a injection molding machine. Such ram type or plunger type of injection molding machine have traditionally been utilized with pellets. Accordingly, one of differences with a pellet machine is that the pellets are fed to the barrel through a hopper and then the plunger liquified the pellets by the pressure applied to the pellets and then forces them into a mold. Because of the variance in the voids between the pellets, this results in discrepancies in the shots fed into the mold. Accordingly, it was highly desirable to have a mechanism in the plunger injection molding machine essentially for controlling the size or maintaining the size of the shot of liquid composition at the same level for each molding operation. It is also desirable to have a mechanism in the plunger injection molding machine which would control or turn on and turn off the pressure from the feed means of the liquid composition into the injection molding machines from the pressure that is applied by the plunger. It was also desirable to have a means for sealing the plunger so as to prevent the liquid composition that was in front of the plunger from leaking past the plunger and out of the rear of the barrel means in a plunger injection molding machine.
Accordingly, these modifications are shown in the enclosed figures.
Accordingly, it is one object of the present invention to provide for a plunger injection molding machine suitable for molding liquid compositions that are fed into the machine.
It is an additional objection of the present invention to provide for a plunger injection molding machine which has a sealing means for effectively sealing the front end of the plunger means so as to prevent liquid composition for escaping from the rear barrel means in the injection molding machine.
It is an additional object of the present invention to provide a stop means for controlling and regulating the rearward motion of the plunger means from molding operation to molding operation.
It is yet an additional objection of the present invention to provide a means in a liquid plunger injection molding machine for maintaining a shot size inserted by the plunger into the mold the same for molding operation to molding operatopm of desired.
It is yet still another object of the present invention to provide for a valve on a feed means for feeding the liquid composition such that the valve means completely turns on and off the pressure and the flow of liquid composition from the feed means into the barrel means of a plunger type of injection molding machine.